A Thai-trained lawyer, Nuon Chea rose quickly through the ranks of the Khmer Rouge, as it grew from a small Maoist rebel group to a force capable of taking over the country.

Analysts say he had an important decision-making role in the regime, which instituted radical policies aimed at creating an agrarian utopia, but in reality caused the deaths of more than a million people through hunger, illness, overwork and execution.

Nuon Chea himself has consistently denied any responsibility for the deaths, but earlier this year he indicated he was ready to face the tribunal.

After many long delays, the UN-backed trials are finally expected to begin next year.

Only one other suspect, Kang Kek Ieu - also known as Duch - has so far been detained.

Duch, who was arrested in July, was in charge of the notorious S21 jail in Phnom Penh, where more than 17,000 men, women and children are thought to have been imprisoned and brutally tortured.

Four other people are said to be under investigation.

Their names have not been revealed, but are thought to include former president Khieu Samphan - who has been living next door to Nong Chea in Pailin - and Khmer Rouge foreign minister Ieng Sary.

Survivors have welcomed the charges against Nong Chea and Duch, but they have also expressed doubts about whether these elderly leaders will ever be brought to account for their deeds during the Khmer Rouge years.

It is already too late to try Pol Pot, and the regime's military commander and one of Pol Pot's most ruthless henchmen, Ta Mok, died last year.